CAPT OF AN MLG DSNDED BELOW A SAFE ALT WHILE MAKING A VISUAL CIRCLE TO LAND APCH DUE TO A MIS-SET ALTIMETER ALERTER.

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IT WAS NIGHT. WE WERE ON A L DOWNWIND AT SPOKANE AT 5000 FT MSL AND WERE CLRED FOR A VISUAL APCH FOR RWY 21. WE WERE REQUESTED TO TURN BASE QUICKLY AS POSSIBLE BECAUSE OF TFC INBOUND STRAIGHT IN. AS WE WERE DSNDING DOWNWIND I NOTICED THAT WE WERE GETTING RATHER LOW AND I WAS EXPECTING THE CAPT TO TURN BASE FOR OUR SHORT APCH. I GLANCED AT THE RADIO ALTIMETER AND SAW WE WERE AT 900 FT AGL DSNDING. BEFORE I SAID ANYTHING THE CAPT REALIZED HE WAS TOO LOW FOR OUR POS ON DOWNWIND. HE LEVELED OFF (AT APPROX 700 AGL) AND BEGAN A SLOW CLB AND TURNED BASE LEG. AFTER WE LANDED UNEVENTFULLY; WE TALKED ABOUT WHAT HAPPENED AND HE SAID HE HAD SET 2000 FT IN THE ALT ALERTER (I HAD JUST NOTICED THIS) FOR THE VISUAL APCH. THE FIELD ELEVATION IS 2372 FT MSL. IN HIS MIND HE HAD PLANNED DSNDING TO 2000 FT MSL EVEN THOUGH WHEN WE HAD BRIEFED TO APCH HE HAD BRIEFED THE ARPT ELEVATION AND MSA FROM THE APCH CHART. I THINK THE SIT WAS COMPOUNDED BECAUSE HE WAS NOT WATCHING THE ARPT OFF ON HIS L SIDE AND LETTING THE AUTOPLT DSND. I COULD HAVE BEEN MORE QUESTIONING ON HIS SHORT APCH TECHNIQUE. I ALSO MISSED NOTING WHAT HE SELECTED ON THE ALT ALERTER WHICH WOULD HAVE RAISED QUESTIONS ON HIS INTENTIONS.